Model operation behaviour using activity diagram

[dave keeling]

 I am using EA to document existing C code structure and behaviour. I have imported the code modules which has worked successfully to create classes representing the modules (header and source). The functions and variables are well represented in the model as operations and attributes of the class which is good.
However to document the behaviour implemented by an operation I would like to add an activity diagram, I can add a textual behaviour specification in the properties of the function but not a diagram.
 
If have found that it is possible to add a callOperation action to an activity diagram and to create an activity diagram under the action; however this is not ideal as it relates to the use of the operation (C function) rather than to its specification. I have used other UML tools that allow an activity diagram to be added under an operation within a class.